Conventional quantum cascade lasers (QCLs) are composed of a superlattice of quantum wells (QWs) and barriers of fixed composition. That is, the composition of the QWs is the same and the composition of the barriers is the same. For conventional QCLs optimized for high continuous-wave (CW) power and emitting in the 4.6 to 4.8 μm range, there is substantial thermally activated carrier leakage from the upper laser level to the continuum. The leakage is evidenced by highly temperature sensitive electro-optical characteristics at and above room temperature (RT). That is, the characteristic temperature coefficient, T0, for the threshold-current density, Jth, is found to have relatively low values of ˜140 K for 4.6 μm emitting devices (ideal T0 values are ˜300 K). See A. Lyakh, C. Pflugl, L. Diehl et al., Appl. Phys. Lett. Vol. 92, p. 111110, 2008; Y. Bai, S. R. Darvish, S. Slivken, et al., Appl. Phys. Lett., Vol. 92, p. 101105, 2008; D. Botez, J. C. Shin, L. J. Mawst et al., Proc. SPIE, Vol. 7616, 76160N, February 2010. The characteristic temperature coefficient, T1, for the slope efficiency, ηd, is also found to have relatively low values of ˜140 K for 4.6 μm emitting devices over the 20-90° C. temperature range. See A. Lyakh, C. Pflugl, L. Diehl et al., Appl. Phys. Lett. Vol. 92, p. 111110, 2008. Thus, even for buried-heterostructure devices; that is, structures with optimal heat removal, the strong temperature sensitivity of their electro-optical characteristics has not allowed for optimal performance. For instance, at 4.6 μm the best results at RT are 12.7% maximum CW wallplug efficiency, far short of the theoretically predicted upper limit of 28%. See A. Lyakh, R. Maulini, A. Tsekoun, et al., Appl. Phys. Lett. Vol. 95, p. 141113 (2009); J. Faist, Appl. Phys. Lett. Vol. 90, p. 253512 (2007). As the emission wavelength decreases below 4.5 μm, the problem of thermally activated carrier leakage becomes even more challenging. Conventional QCLs operating below 4.5 μm exhibit T1 values as low as 29 K and RT CW wallplug efficiencies of 1%. See J. S. Yu et al., Appl. Phys. Lett., 88, 251118 (2006); J. S. Yu et al., Appl. Phys. Lett., 88, 041111 (2006).
Some state-of-the-art QCLs have been developed which exhibit lower carrier leakage, higher T0 and T1 values and improved RT maximum CW wallplug efficiencies. See, e.g., Shin, J. C. et al., Ultra-low temperature sensitive deep-well quantum cascade lasers (λ=4.8 μm) via uptapering conduction band edge of injector regions, Electronics Letters, Jul. 2, 2009, Vol. 45, No. 14. These state-of-the-art QCLs are InP-based devices composed of a superlattice of QWs and barriers in which the composition of the QWs varies and the composition of the barriers varies, allowing for strategically positioned deeper (in energy) quantum wells and taller (in energy) barriers. However, the range of compositions and thicknesses that can be accessed for the quantum wells and barriers is limited by strain-thickness considerations in order to avoid strain relaxation in the many layers (500-900 or more) of highly strained materials that are needed in practical devices. As the critical thicknesses for strain relaxation are approached, device reliability is likely to deteriorate through thermally activated relaxation processes. Thus, the emission wavelength for these state-of-the-art QCLs remains in the range of about 4.6 to 4.8 μm.
Some InP-based devices have been developed which make use of highly strained materials for the quantum wells and barriers of the superlattice. See, e.g., Bandyopadhyay, N. et al., Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ˜3.76 μm, Applied Physics Letters, 97, 131117 (2010); Bismuto, M. et al., High power Sb-free quantum cascade laser emitting at 3.3 μm above 350 K, Applied Physics Letters, 98, 191104 (2011). Although some of these devices may achieve short wavelength emission, the performance of the devices is poor as evidenced by low T0 and T1 values and low CW powers and efficiencies (or only pulsed operation). In addition, as noted above, device reliability may deteriorate due to the use of highly strained materials.